pgsql: Improve documentation of the CREATEROLE attibute.

Improve documentation of the CREATEROLE attibute.

In user-manag.sgml, document precisely what privileges are conveyed
by CREATEROLE. Make particular note of the fact that it allows
changing passwords and granting access to high-privilege roles.
Also remove the suggestion of using a user with CREATEROLE and
CREATEDB instead of a superuser, as there is no real security
advantage to this approach.

Elsewhere in the documentation, adjust text that suggests that
<literal>CREATEROLE</literal> only allows for role creation, and
refer to the documentation in user-manag.sgml as appropriate.

Patch by me, reviewed by Álvaro Herrera

Discussion: http://postgr.es/m/CA+TgmoZBsPL8nPhvYecx7iGo5qpDRqa9k_AcaW1SbOjugAY1Ag@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5dac191edf1816860993e5f43b817eafcece1166

Modified Files
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doc/src/sgml/ref/alter_role.sgml | 2 +-
doc/src/sgml/ref/create_role.sgml | 10 ++++-----
doc/src/sgml/ref/createuser.sgml | 18 ++++++++++-----
doc/src/sgml/user-manag.sgml | 47 +++++++++++++++++++++++++++------------
4 files changed, 52 insertions(+), 25 deletions(-)